1. Field of the Invention
The present invention relates to an embedded phase-change memory and a method of fabricating the same, and more particularly, to an embedded phase-change memory that carries out a memory function within a System On Chip (SOC), and a method of fabricating the same.
2. Description of the Related Art
Phase-change memories are non-volatile memories which maintain stored information even without a power supply. The phase-change memory includes a phase-change material whose electrical resistance changes depending on its crystalline structure. The phase-change material generally consists of elements of Groups VIA through IVA on the periodic table. Because the crystalline structure and electrical resistance remain regardless of the supply of electrical power, this memory is non-volatile.
The phase change which results in a difference of electrical resistance is caused by an electric current. The resistance is high when the crystalline structure of the phase-change material is amorphous, and low when the structure is crystalline. The difference between the two states is at least 100-fold. The phase change from the amorphous state to the crystalline state is referred to as “Set,” and the phase change from the crystalline state to the amorphous state is referred to as “Reset.” The electrical input can both “Set” and “Reset,” allowing the phase change material to be used in non-volatile memory.
Ge2Sb2Te5(GST) is a common phase-change material. When a current pulse flows through GST under the amorphous state for a certain time period, the temperature of the GST is raised to over its crystallization temperature, thereby changing the state of the GST into the crystalline state (“Set”). By applying a current pulse to the GST under the crystalline state, the temperature of the GST is raised to over its melting point, so that the GST is in a liquid state. When the current is stopped, the amorphous crystalline structure of the liquid state remains (“Reset”).
To “Reset,” the phase-change material has to be melted and the amorphous state kept by quenching, so a high current pulse has to be applied for a short time, compared with the case of “Set.” Generally, a unit phase-change memory device includes a phase-change material and a heating layer. Because the phase-change material alone cannot generate sufficient heat to cause the phase change, the heating layer is composed of a material with a high electrical resistance and contacts the phase-change material, to promote the heat generation.
Conventional phase-change memory is fabricated using CMOS technology. A unit cell consists of a single phase-change memory device and a single cell transistor. Current pulses are applied through a bit line, and a cell transistor is turned on when a voltage higher than its threshold voltage is supplied to a word line. By turning on the cell transistor, the phase-change memory device is connected to the bit line through which the current pulses are supplied. The current pulses supplied at this time perform “Set” and “Reset” of the phase-change memory device.
Since a high “Reset” current is needed, the cell transistor must be a bipolar transistor or a MOS transistor with a significantly wide gate. A vertical-structured pnp-type bipolar transistor can secure a high power density with a cell size of only 5F2˜8F2, but is not often used due to difficulty in processing. Accordingly, most cell transistors are MOS transistors.
In order to drive the phase-change memory device using the MOS transistor, the gate width has to be sufficient. Because such a MOS transistor occupies a large area on a silicon substrate, current research is directed toward decreasing the reset current to reduce the gate width, thereby improving integration.
Memories are classified into stand-alone memory and embedded memory. Embedded memory is differentiated from stand-alone memory by incorporating logic to perform its function within a single chip, and is one component of a System-On-Chip (SOC). By directly connecting a microprocessor to an embedded memory within an SOC, bandwidth can be increased while decreasing power consumption.
As silicon semiconductor processing techniques have developed, the functions embodied by SOCs have widened into diverse fields such as mobile communications and multimedia. The demand for chips for graphics, audio and video applications is increasing greatly. Therefore, more information must be processed in the SOC than ever before, and thus the role of embedded memory in the SOC is becoming more and more important. Increasingly high performance of embedded memory will be needed as SOCs become more complicated.
DRAM, SRAM, and FeRAM (Ferroelectric Random Access Memory) may be considered for a unit cell of the embedded memory. However, DRAM and SRAM are volatile, losing stored information when power is turned off, and FeRAM needs fastidious fabrication for a reliable device. Accordingly, an embedded memory is needed for embodying high performance and multiple functions.